In existing wireless technologies, signal repeating devices, or “repeaters” are used to extend the coverage of the overall wireless system. For example, often such systems consist of a plurality of base stations that communicate with each other and operate to provide a defined coverage area. In such coverage areas, there are often smaller areas that have very low signal reception, such as areas within buildings or areas that are otherwise obstructed. Rather than implementing another costly and large base station to provide coverage to such low signal areas, repeaters are utilized. A repeater essentially has a donor antenna that is in communication with one or more base stations. The repeater receives downlink signals from the base station, processes and amplifies those signals, and then transmits those signals through a coverage antenna into the area that otherwise has low signal reception or low signal power.
For example, referring to FIG. 1, a basic wireless communication system 10 might include a base station 12 that communicates with a repeater 14 having a donor antenna 16, a coverage antenna 18, and processing electronics 20 that are positioned between the antennas to process and amplify the repeated signal. Accordingly, downlink wireless signals 22 are received by the donor antenna 16 of the repeater, and are then repeated through the coverage antenna 18 as downlink signals 22a. The downlink signals 22a are received by one or more wireless communication devices, such as cell phones 24. Similarly, in an uplink direction, as indicated by reference numerals 26 and 26a, the wireless device 24 communicates signals back to the coverage antenna and the repeated signal 26 is then provided as an uplink signal back to the base station 12. As would be readily understood by a person of ordinary skill in the art, such repeater devices 14 can take many different forms.
One performance-limiting characteristic for a repeater is the isolation between the two opposing antennas or sets of antennas. The isolation limits the gain that may be implemented within the repeater 14. The gain generally has to be less than the isolation to ensure stability. For example, if the isolation between the antennas 16, 18 of the repeater 14 is around 60 dB, then the maximum system gain allowed might be around 45-50 dB, which would allow for a 10-15 dB gain margin.
One particular issue to be addressed within a repeater is the feedback signal that comes from the coverage antenna 18 back to the donor antenna 16 (or vice-versa for the uplink traffic). Such feedback signals may come directly from the transmitting antenna as indicated in FIG. 1 as signal 30 or indirectly from reflectors as signal 32, which could be caused by direct coupling between the antennas 16 and 18 or via reflections on surfaces close to the repeater as indicated by reference numeral 32 in FIG. 1. Because the feedback signals 30 and 32 in an on-frequency repeater case are generally the same frequency (neglecting the frequency shift due to the Doppler Effect in case of relative movement between reflectors and repeater antennas) as the input signal 22, it cannot be removed by conventional frequency domain filtering techniques.
To remove the feedback signals 30, 32 without corrupting the desired uplink or downlink signals 22, 26, the feedback signals must be subtracted from any received signal or input signal. Often, the time delay of the signal through the repeater path is long enough to statistically de-correlate the echo from the input signal without affecting system performance. In such a case, adaptive filtering algorithms can be used to attenuate the feedback signal from the repeater. For example, in some existing repeater products, such as the Node C/M/G as provided by Andrew Corporation, A CommScope Company, Hickory, N.C., the symbol rate of the received signal or input signal is fast enough so that the repeater delay is sufficiently long to de-correlate the feedback signal from the incoming signals. However, such a technique for addressing feedback signals is not suitable for all repeaters.
In the case of repeaters that support narrowband signals with a symbol period which exceeds the value of tg/2 (half of the group delay of the repeater), the delay through the repeater is typically not long enough to de-correlate the input signals from the echo signals. As such, a time delay-based adaptive filtering algorithm is not able to distinguish sufficiently between the desired input signals, such as the downlink signals from the base station, and the undesired feedback signal between the transmit and receive antennas of the repeater. For example, for narrowband standards, the date rate or symbol rate through the channel is sufficiently slow, such that it would take around 50-100 microseconds of delay in the repeater to sufficiently de-correlate the feedback signal from the desired input signal. Such an amount of delay could be too high, and could cause intra- or intersymbol interference that could not be tolerated by wireless devices, such as mobile phones, in various overlap areas where the wireless devices can receive signals at similar levels from both the base station and a repeater.
Accordingly, existing techniques in the art that incorporate time delays are unable to provide the necessary cancellation of feedback signals in a repeater system that support signals with a symbol period exceeding half of the repeater group delay.